Porous materials capable of being prepared utilizing biological products such as proteins, sugars, etc. are utilized in a wide field in industry, inclusive of a medical field of wound covering material, hemostatic sponge, controlled drug release carrier, retractor, etc., a field of daily living necessaries such as paper diapers, sanitary napkins, etc., a field of water purification where such materials can be applied as a support serving as a den of microorganisms, bacteria, etc., a field of cosmetics or beauty treatment aiming at moisturizing or the like through the use by a beauty salon or an individual, a cell culture support or a tissue regeneration support in the tissue engineering or regenerative medical engineering, and the like.
As such a biological product constituting a porous material, there are known sugars such as cellulose, chitin, etc.; and a group of proteins such as collagen, keratin, fibroin, etc.
Among them, collagen has been most frequently utilized as the protein; however, it has become very difficult to utilize bovine-derived collagen since the BSE problem emerged. Furthermore, as for pig-derived collagen, there is involved a problem of new infectious diseases, and as for fish-derived collagen, there is involved a problem of strength of the porous material, so that it is difficult to put it into practical use. In addition, though keratin is obtainable from wool or feather, there is involved a problem of availability of raw materials, so that it is difficult to industrially utilize keratin. As for the wool, raw material prices are rising dramatically, and as for the feather, there is no marketplace, so that it is not easy to obtain raw materials. On the contrary, as for fibroin, it is possible to easily obtain fibroin from silk, and from the viewpoint of acquisition of raw materials, it can be expected that fibroin is stably supplied, and its price is stable, and hence, it is easy to industrially utilize fibroin.
Moreover, in addition to a clothing application, fibroin has a tract record that it has been long used as a surgical suture, and nowadays, fibroin is also utilized as additives for foods and cosmetics and is free from a problem regarding safety on the human body. Therefore, fibroin is sufficiently applicable to the utilization fields of porous materials as described above.
As for a technique for preparing a silk fibroin porous material, there are some reports. For example, there is proposed a method in which a fibroin aqueous solution is quickly frozen and then dipped in a crystallization solvent, and thawing and crystallization are allowed to simultaneously proceed, thereby producing a porous material of fibroin (Patent Document 1). However, according to this method, it is necessary to use a large amount of an organic solvent that is the crystallization solvent, and furthermore, a possibility of retention of the solvent may not be denied, so that there is involved a problem in the use of the method in the above-described application fields such as the medical field, etc.
In addition, there is proposed a method in which a fibroin aqueous solution is gelated while keeping it at a pH of not more than 6, or a poor solvent is added to that aqueous solution to achieve gelation, and the resulting gel is freeze-dried, thereby producing a porous material of fibroin (Patent Document 2). However, according to this method, it may be impossible to obtain a porous material with sufficient strength.
Furthermore, there is proposed a method in which after freezing a fibroin aqueous solution, its frozen state is kept for a long period of time, thereby producing a porous material (Patent Document 3). However, according to investigations made by the present inventors, this technique is poor in reproducibility, and in many cases, the porous material may not be prepared.
On the other hand, there is reported a method in which a porous material of fibroin with high strength is obtained surely and simply and easily as compared with the foregoing preparation techniques of a silk fibroin porous material (Patent Document 4 and Non-Patent Document 1). In Patent Document 4 and Non-Patent Document 1, it is disclosed that after adding a small amount of an organic solvent to a fibroin aqueous solution, the contents are frozen for a certain period of time and then thawed, whereby a hydrogel having a high water content and excellent mechanical strength may be produced.